Indoles are industrially useful substances as a starting material for the preparation of perfumes, amino acids such as tryptophane, and stabilizers for high polymers. It is known that indoles are prepared by reacting anilines and 1,2-glycols in the presence of a catalyst. Various catalysts have been proposed for this purpose, including a dehydrogenation catalyst containing Cu, Cr, Co, Fe, Ni, Zn, Mn, Al, Ca, Pd, Pt, Rh, etc., which are commonly used in the dehydrogenation of alcohols (Japanese Patent Application (OPI) No. 36451/81 (the term "OPI" as used herein means a "published unexamined Japanese patent application")), a cadmium sulfate and/or zinc sulfate-containing catalyst (Japanese Patent Application (OPI) No. 53652/81), a cadmium chloride and/or zinc chloride-containing catalyst (Japanese Patent Application (OPI) No. 46865/81), a copper-containing catalyst (Japanese Patent Application (OPI) No. 55366/81), a cadmium sulfide and/or zinc sulfide-containing catalyst (Japanese Patent Application (OPI) No. 169668/81), a cadmium chloride catalyst (Japanese Patent Application (OPI) No. 109471/83, a calcium sulfate-containing catalyst (Japanese Patent Application (OPI) No. 121270/83), and a silver oxide-containing catalyst (Japanese Patent Application (OPI) No. 225062/83).
In addition, Japanese Patent Application (OPI) No. 46067/83 discloses a method in which the catalyst reaction of anilines and 1,2-glycols is carried out in the presence of water and hydrogen. It is described that numerous catalysts can be used in this method and as one of the catalysts, cadmium halide is disclosed. However, there can be found no description that cadmium iodide can be acutally used nor experiment demonstrating that cadmium iodide can be used.
The above known methods except for those of Japanese Patent Application (OPI) Nos. 46067/83 and 109471/83 are all limited to a gas-phase reaction. In the methods of Japanese Patent Application (OPI) Nos. 46067/83 and 109471/83, it is disclosed that any of gas-phase, liquid-phase, and gas/liquid mixed-phase reactions can be employed. In fact, however, the concrete examples disclosed therein are all directed to the gas-phase reaction, and thus it is not evident whether or not the liquid-phase reaction can be employed.
Japanese Patent Application (OPI) Nos. 73060/81 and 110672/81 disclose a process for preparing indoles in the liquid phase using a catalyst containing a metal of Group VIII in the Periodic Table, magnesium oxide, and the like. However, the results of our experiments on the prior art showed that the process of our experiments on the prior art showed that the process is unsatisfactory for industrial use since large amounts of by-products are formed and the yield is not sufficiently high.
Known catalysts for use in the preparation of indoles by the gas-phase reaction have various disadvantages. For example, their catalyst activity is seriously decreased during the reaction. For the recovery of catalytic activity, therefore, it is necessary to frequently apply a complicated catalyst regeneration operation. Some catalysts cannot be regenerated and activated. Silver-containing catalysts are expensive. Therefore, these catalysts are unsuitable for practical use from a viewpoint of production costs.